When an engine undergoes combustion in an oxygen-rich atmosphere, carbon monoxide, hydrocarbons and nitrogen oxides (NOx) are contained in the exhaust gas. Selective catalytic reduction (SCR) catalysts are known as catalysts that reduce NOx discharged in an oxygen atmosphere using a reducing agent such as ammonia.
Patent Document 1 describes a catalyst for selective catalytic reduction of nitrogen oxides that is characterized in containing zeolite or a zeolite analog containing 1 wt % to 10 wt % of copper based on the total weight of the zeolite or zeolite analog, and a homogeneous cerium-zirconium mixed oxide and/or cerium oxide, which improves NOx purification rate at a temperature of 350° C. or higher (Patent Document 1, claim 1, paragraph [0015]).
Patent Document 2 describes a method for producing a catalyst for exhaust gas purification by loading ceria onto a metal oxide other than the ceria, characterized in comprising a step for preparing a mixed solution containing a cerium complex, which contains cerium and a ligand coordinated on the cerium, and an organic solvent for dissolving the cerium complex, and a step for drying the mixed solution and firing the resulting product, wherein the average particle diameter of the ceria is 5 nm or less (Patent Document 2, claim 1, claim 5).